the collector has optimizations to avoid allocation and reslicing
during the common case of searching for top hits
however, in some cases users request an a very large number of
search hits to be returned (attempting to get them all) this
caused unnecessary allocation of ram.
to address this we introduce a new constant PreAllocSizeSkipCap
it defaults the value of 1000. if your search+skip is less than
this constant, you get the optimized behavior. if your
search+skip is greater than this, we cap the preallcations to
this lower value. additional space is acquired on an as needed
basis by growing the DocumentMatchPool and reslicing the
collector backing slice
applications can change the value of PreAllocSizeSkipCap to suit
their own needs
fixes#408
60efecc8e9